Researchers develop a way to funnel solar energy

September 12, 2010
by Anne Trafton

This filament containing about 30 million carbon nanotubes absorbs energy from the sun as photons and then re-emits photons of lower energy, creating the fluorescence seen here. The red regions indicate highest energy intensity, and green and blue are lower intensity. Image: Geraldine Paulus

(PhysOrg.com) -- Using carbon nanotubes (hollow tubes of carbon atoms), MIT chemical engineers have found a way to concentrate solar energy 100 times more than a regular photovoltaic cell. Such nanotubes could form antennas that capture and focus light energy, potentially allowing much smaller and more powerful solar arrays.

"Instead of having your whole roof be a photovoltaic cell, you could have little spots that were tiny photovoltaic cells, with antennas that would drive photons into them," says Michael Strano, the Charles and Hilda Roddey Associate Professor of Chemical Engineering and leader of the research team.

Strano and his students describe their new carbon nanotube antenna, or "solar funnel," in the Sept. 12 online edition of the journal Nature Materials. Lead authors of the paper are postdoctoral associate Jae-Hee Han and graduate student Geraldine Paulus.

Their new antennas might also be useful for any other application that requires light to be concentrated, such as night-vision goggles or telescopes.

Solar panels generate electricity by converting photons (packets of light energy) into an electric current. Strano's nanotube antenna boosts the number of photons that can be captured and transforms the light into energy that can be funneled into a solar cell.

The antenna consists of a fibrous rope about 10 micrometers (millionths of a meter) long and four micrometers thick, containing about 30 million carbon nanotubes. Strano's team built, for the first time, a fiber made of two layers of nanotubes with different electrical properties — specifically, different bandgaps.

In any material, electrons can exist at different energy levels. When a photon strikes the surface, it excites an electron to a higher energy level, which is specific to the material. The interaction between the energized electron and the hole it leaves behind is called an exciton, and the difference in energy levels between the hole and the electron is known as the bandgap.

The inner layer of the antenna contains nanotubes with a small bandgap, and nanotubes in the outer layer have a higher bandgap. That's important because excitons like to flow from high to low energy. In this case, that means the excitons in the outer layer flow to the inner layer, where they can exist in a lower (but still excited) energy state.

Therefore, when light energy strikes the material, all of the excitons flow to the center of the fiber, where they are concentrated. Strano and his team have not yet built a photovoltaic device using the antenna, but they plan to. In such a device, the antenna would concentrate photons before the photovoltaic cell converts them to an electrical current. This could be done by constructing the antenna around a core of semiconducting material.

The interface between the semiconductor and the nanotubes would separate the electron from the hole, with electrons being collected at one electrode touching the inner semiconductor, and holes collected at an electrode touching the nanotubes. This system would then generate electric current. The efficiency of such a solar cell would depend on the materials used for the electrode, according to the researchers.

Strano's team is the first to construct nanotube fibers in which they can control the properties of different layers, an achievement made possible by recent advances in separating nanotubes with different properties.

While the cost of carbon nanotubes was once prohibitive, it has been coming down in recent years as chemical companies build up their manufacturing capacity. "At some point in the near future, carbon nanotubes will likely be sold for pennies per pound, as polymers are sold," says Strano. "With this cost, the addition to a solar cell might be negligible compared to the fabrication and raw material cost of the cell itself, just as coatings and polymer components are small parts of the cost of a photovoltaic cell."

Strano's team is now working on ways to minimize the energy lost as excitons flow through the fiber, and on ways to generate more than one exciton per photon. The nanotube bundles described in the Nature Materials paper lose about 13 percent of the energy they absorb, but the team is working on new antennas that would lose only 1 percent.

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Efficiency has nothing to do with this article. Efficiency only uses energy input and usable energy output to make a ratio, which has nothing to do with area of the solar array. Efficiency deals with how well the light input is converted into a flow of electrons.

There is no debate over solar energy technology that is anything other than blather. The simple fact is that humanity requires technologies that consume ever increasing energy flux densities in order to survive and progress. Solar is good for plants. We could use some more plants in the deserts by building water projects like the North American Water an Power Alliance plan engineered by Parsons way back when. But humanity requires power that will come from energy dense plasmas in the future. Some from power stations that colonist will build on the moon.

"Instead of having your whole roof be a photovoltaic cell, you could have little spots that were tiny photovoltaic cells, with antennas that would drive photons into them," says Michael Strano, ..."

Yet, the fiber of nanotubes has to cover the same area or half the area (if 2x as efficient) to generate the same power as the traditional solar panels. The key question is whether the fiber of nanotubes can be made cheaper than a silicon solar cell.

Modernmystic - I have to disagree - solar will be a significant part of our energy future - here is just on example of a solar plant that will generate enough power for 800,000 homes. http://www.nytime...858.html We may never totally run our civilation on solar alone - but I disagree with the term "dead end technology"

You should check your numbers again Modernmystic. 1m^2 at the earth surface receives about 1000watts. A 1km square installation will be receiving just about 1GW of power during the sunny hours of the day. 1GW*8hrs=8GWhours/day. 8GWhours*.14(solar cell efficiency) and our 1km^2 installation is netting 1.12 GWhours/day. The average american household uses about 30kWhours/day. Which means our installation could power 30000 homes.

Now you may be thinking to yourself "but what about storage? Your numbers are all well and good during the sunny hours of the day, but I like to surf the web at night. How am I going to do that if the sun isn't out?"

Great question Modernmystic :) There are many energy storage options available to modern man, here are 2: We could pump water up to a higher elevation and let it run downhill at night.We could heat something up REALLY hot and use the excess thermal energy to run steam turbines.

Or we could use the energy to harvest CO2 from the air and make our own hydrocarbons.

The simple fact is there is no greater power source available to us than solar power. 89petaWatts hit the earth's surface every second. There does not exist enough fissionable material on earth to generate as much power as hits the earth every second. We could start making fusion powerplants from now until forever and we won't be able to keep up with our energy needs in the future. Solar may very well be the ONLY viable energy source at the civilization level.

I look forward to the day in the near future when internet commentators such as modernmystic and others will be saying: Fusion power is a great niche power source, but you're never going to be able to run a civilization on it. Until then check out Daniel Nocera's Pop tech talk: